Solenoid construction

ABSTRACT

High-pressure and low-pressure AC push solenoid apparatus and method and pull solenoids of the armature in tube type for operating hydraulic valves. The solenoids include unique overall structural configurations and also improved armature tubes, armatures, push pins, yokes and a preferred range for the ratio of coil length to working gap lengths, providing high-force and stabilized operating temperatures.

United States Patent [7 21 Inventor Richard Thompson Fort Wayne, Ind.

121] Appl. No. 30,170

|22| Filed Apr. 20, 1970 145] Patented ,Ian. 4, 1972 I73] Assignee G. W. Llsk Company, Inc.

Clifton Springs, N.Y.

[54] SOLENOID CONSTRUCTION 28 Claims, 9 Drawing Figs.

[52] U.S. Cl 335/255, 335/279 [51] Int. Cl 1101f 3/00 [50] Field of Search 335/255, 138, 236, 251,132, 278, 260, 261 271, 281, 282, 279

[56] References Cited 7 UNITED STATES PATENTS 3,206,656 9/1965 Musgrave. 335/255 3,326,511 6/1967 Hallgreen 335/255 2,924,685 2/1960 Burch 335/132 2,899,609 8/1959 Andrews 335/255 3,519,967 7/1970 Mullen 335/132 3,166,692 1/1965 Forrester.... 335/251 3,381,181 4/1968 Weathers 335/251 3,221,191 11/1965 Cuches 335/281 3,273,095 9/1966 Rossenbeek 335/251 Primary Examiner.l. D, Miller Assistant Examiner-R. Skudy Att0rneySchovee & Boston ABSTRACT: High-pressure and low-pressure AC push solenoid apparatus and method and pull solenoids of the armature in tube type for operating hydraulic valves. The solenoids include unique overall structural configurations and also improved armature tubes, armatures, push pins, yokes andva preferred range for the ratio of coil length to working gap lengths, providing high-force and stabilized operating temperatures.

PATENTEO JAN 41972 SHEET 1 BF 2 FIG. 3

FIG. 1

FIG. 4

INVENTOR. RICHARD THOMPSON yw v KW ATTORNEYS PAIENTED JAN 4 I972 SHEET 2 BF 2 FIG. 7

INVENTOR. RICHARD THOMPSON BYXMLKW ATTORNEYS BACKGROUNDOF THE INVENTION 1, Field of the Invention The present invention relates to AC solenoids and more particularly to solenoids of the armature-in-tube type for use with hydraulic valves.

2. Description of the Prior Art Two generaltypes of solenoids for operating hydraulic valves areknown'. ln suc hsolenoids, the solenoidcoils are usually sealed from the fluid, and the fluid must-be prevented from leaking out from the solenoid-valve assembly.

A firstknown type of solenoid employs a solenoid frame and armature constructed from magnetic steel laminations riveted together. Because this construction cannot seal off system pressurefrom the inside to the outside of the valve, an O-ring isused as a dynamic, seal against the solenoid push pin. About percent of the solenoid force islost in having to overcome the drag force of the O-ring on the solenoid pushpin; this drag force increases greatly as the operating fluid pressure increases and in some cases the drag force may be higher than the solenoid force necessitating that the fluid pressure be reduced to a fraction of the operating pressure, in order that the solenoid will be capable offoperating the hydraulic valve. The laminated construction does not hold up well under impact loads; it will not provide a seal when operating a hydraulic valve; and the O-ring seal is a constant source of friction and leakage. Further, the O-ring seal often fails (by beginning to leak) after about 4 million cycles, requiring replacement by a new O-ring seal that is usually difficult to install. Further, the installation procedure requires that the hydraulic valve be taken out of service and disassembled during replacement of the seal.

A second known type of solenoid (to which type the present invention relates) offers certain advantages over the abovediscussed first type by virtue of the second type being built around an armature tube containing a solid annature in oil, thus eliminatingthe need for, and the disadvantages attendant on the use of, a dynamic O-ring seal. However, this second type of solenoidis subject to serious drawbacks in that it is usually associated with relatively low force levels and is generally plagued by excessiveheating due to circulating eddy currents in the solid metal partsof the magnetic circuit and due to thesolenoid always having a relatively high holding current (actually an inrush current) due to the nonmagnetic armature tube being in the path of the magnetic lines of force. Further, this second type of solenoid, due to its very low force output can be used only to operate the smallest of valves, unless a pilot valve is used to operate the main valve, however, using such a pilot valve adds considerable complication and expense to the assembly. The solenoid of the present invention eliminates the above-mentioned problems of the known second type of solenoid.

The present invention is of the above-described second type of solenoid. For purpose of the present specification and claims the above second type of solenoid is hereby defined as, andwill hereinafter be referred to as, the armature-in-tube type of solenoid. The solenoids. of the present invention have all of the advantages of the prior art armature-in-tube solenoids, plus they also have force levels and stabilized temperatures comparable to those of the fully laminated, abovedescribed first type of solenoid.

SUMMARY. OF THE PRESENT INVENTION The improved solenoids of the present invention overcome the disadvantages inherent in both of the above-discussed types of prior art solenoids.- The solenoids of the present in vention employ many rigid, nonlaminated parts which hold up well under impact, and slotted solid parts to reduce heating due to circulating eddy currents. Both the high-pressure push and the high-pressure pull solenoids of the present invention employ an improved, one-piece semiaustenitic armature tube treated to have magnetic and nonmagnetic sections to 2 eliminate high holding current consumption. fidth'of the lowpressure solenoids of the present invention (both the'piish arid the pull) employ a very thin, very high SllCtlglh,-fill niag'iie'tic armature tube. The push solenoids (both the higli and low pressure) further employ an improved'yoke, armature, pushpin, and a particular range for'the'ratio of coil length to working gap length.The pull'solenoid's (both lowpressure' andihigh pressure) employ acapped armature"tube,* improved yokes, and improved armatures.

BRIEF DESCRIPTION OF TIIE DRAWINGS The present invention will be more 'fully understood by reference to the following detailed'desc'ription' thereof, when read in conjunction with the attached drawings, whereinlike reference numerals refer tolike'elementsan'd wherein:

FIG. 1 is a partial cross-sectional, partial diagrammatic block diagram, of a push solenoid l2'ofthe present invention shown connected to a hydraulic valve; I

FIG. 2 is a perspective view of an improved yoke '16 of the solenoid 12 of FIG. 1;

FIG. 3 is a perspective'view of an improved stop 28 of the solenoid 12 of FIG. 1;

FIG. 4 is an enlarged cross-sectional view through the manual override 30 of the solenoid l2 of FIG. 1;

FIG. 5 is a cross-sectional view through animproved pull solenoid of the present invention;

FIG. 6 is a perspective view of the yoke 102 of the solenoid 100 of FIG. 5;

FIG. 7 is a cross-sectional view through another preferred pull solenoid of the present'invention.

FIG. 8 is a perspective, partially cutaway view of I the preferred solenoid armature 166 of the solenoid 150 of FIG. 7; and

FIG. 9 is a perspective, partially cutaway view of another embodiment of an armature l94'useful inthe solenoid 150' of FIG. 7.

DETAILED DESCRIPTION oF iIE PREFERRED EMBODIMENTS For ease of understanding, the following detailed description of the various aspects" of the presentinvention is separated into the two main sections of l) PUSHSOLENOID and (2) PULL SOLENOID. Throughout the present specification and claims, the terms high pressure and low pressure refer to the operating fluid pressure, in pounds per square inch, in a hydraulic valve operated bythe solenoids of the garding the orientation'of the solenoids, the terms stop" and bottom" of a solenoid are h'ereby'defined asfollows. Witli reference to FIG. '5, the-vertically arranged'solenoid I00"'ha's a top 1 and bottom 2; similarly, solenoid 12 of FIG. Ihas atop 3.

and a bottom 4. In each case, thepullpiii 'I68'and thepu'sh pin 24, respectively,

it is therefore the bottom of the'solenoid to which the valve, (e.g. valve 10 of FIG. 1) is connected. These terms are arbitrary in that the solenoids of the present inventionfmayhaveany' desired orientation. The term normal positionof the'arma ture is hereby defined as its unene'r'gi'zed position; as contrasted to the position it occupies'after the' solenoid has been energized, causing the armatureto move through the working gap to its "energized position.

PUSH SOLENOID High-Pressure Push Solenoid With reference to the drawings, FIGS. I thi'ouglid show the: preferred AC, high-pressure push solenoid [2 'of the present invention.

normally extend from the bottom of the solenoid, and

Before describing the details of the solenoid 12, it is believed that a brief description of an overall view of one particular application for the solenoid 12 will be helpful. FIG. 1 shows a fluid valve connected at opposite ends thereof to one of a pair of identical high-pressure push solenoids l2 and 14, made according to the present invention. The valve 10 constitutes a part of a fluid control circuit (not shown), and operation of the valve 10 by either of the solenoids 12 or 14 will produce a certain desired result in the fluid circuit. For example, when the solenoid 12 is energized, it causes a spool 13 in the valve 10 to move to the right (as viewed in FIG. 1) creating fluid communication between certain passages (such as passage 15) in valve 10 and interrupting fluid communication between certain other passages in valve 10. The passages are connected via ports to fluid pressure lines (not shown) of the fluid circuit. The valve 10, the fluid circuit (not shown), and the connection means (not shown) between the valve 10 and the fluid circuit are all well known in the art, fonn no part of the present invention, and therefore need not be described in further detail herein. a

With detailed reference now to the solenoid 12 of FIG. 1, the solenoid 12 comprises a yoke 16 (shown in more detail in FIG. 2) defining a coil window 17, an annular coil 18 positioned in the coil window 17, and a hollow cylindrical armature tube 20 defining an armature cavity 21, positioned inside the yoke 16 and the coil 18. A cylindrical armature 22 is positioned in the armature cavity 21 for axial sliding movement therein. A pushpin 24 is positioned against (and is separate from) the armature 22 and is slidably positioned within an axial bore 26 of a stop 28 (shown in more detail in FIG. 3) connected to the armature tube 20. The stop 28 limits the extent of travel of the armature 22 when the solenoid is energized. The armature 22 is shown in FIG. 1 in its unenergized or normal" position; upon energization of the solenoid 12, the armature 22 moves into contact with the stop 28. A manual override 30 is connected to the left end (as viewed in FIG. I) of the armature tube 20. A layer 32 of epoxy preferably surrounds the various elements of the solenoid.

Detailed reference will now be made to various subassemblies and individual elements of the solenoid 12. FIG. 2 shows the yoke 16 which is of the wound type having a substantially rectangular coil window 17 for accommodating the coil 18. The yoke 16 has a pair of circular openings 36 and 37, through the opposed long sides 38 and 39 thereof respectively, to accommodate the armature tube 20 (see FIG. 1.). In addition, the yoke 16 is provided according to the present invention, with a slot 40 to break up eddy current paths (indicated by arrow 42) and to therefore reduce the circulation of eddy currents that would contribute to the heating of the metal yoke 16. The slot 40 does not interfere with the desired paths (indicated by arrow 44) of the magnetic field. The slot 40 can alternatively be positioned on the opposite side of the openings 36 and 37 from that shown in FIG. 2, and can alternatively be -on both sides of openings 36 and 37, if desired, although in the latter case, mechanical means for holding the two halves of the yoke 16 together would be required.

The coil 18 is of known design and configuration and is connected to lead wires 46 which extend exteriorly of the solenoid 12 in a manner well known to those skilled in the art for connecting the solenoid to a source of electrical energy. The coil 18 can be encapsulated in a protective coating 19 of epoxy resin.

The armature tube 20 seals the coil 18 from the fluid in the solenoid l2, and also forms the cylindrical armature cavity 21,

between the manual override 30 and the stop 28. The tube 20 is made sufficiently thick to withstand the desired operating pressure of the fluid in the armature cavity 21. According to the present invention, the tube 20 is preferably made of semiaustenitic steel, such as that known as 17-7 P.H.

(precipitation hardening) stainless steel. The tube 20 is in its magnetic (martensitic) condition in axial sections 54 and 56 thereof to provide a minimum of hindrance to the magnetic field in passing radially through the wall of the tube 20 in these sections 54 and 56. On the other hand, the tube 20 is in its nonmagnetic (austenitic) condition in axial section 58, to provide the maximum possible hindrance to the portion of the magnetic field trying to pass axially through section 58 of the tube 20. This construction forces as much as possible of the magnetic field to cross a working gap 50 and to thus produce useable solenoid force. Various methods are well known for selectively treating different sections of semiaustenitic steel and such methods can be used to produce the tube 20 of the present invention having sections 54 and 56 in their magnetic condition and having section 58 in its nonmagnetic condition.

Semiaustenitic alloys have austenitic (nonmagnetic) structures in the solution treated condition (annealed condition). The transformation of austenite to martensitic (magnetic) structures can be accomplished by a thermal treatment that conditions the austenite for transformation upon cooling. Cold working (approximately 60 percent reduction) also produces the austenite-to-martensite transformation. My armature tube 20 can be produced starting with tubing in the annealed condition: accomplishing the martensitic transformation of the whole tube by oven heat treating; then selectively annealing one section thereof. An alternative method is to start with cold-worked tubing (already in the martensitic condition) and selectively annealing one section.

The tube 20 of the present invention is much less expensive than the prior art tube comprising two separate pieces, one magnetic and one nonmagnetic, butt-welded together.

The armature 22 is a cylindrical body having a slot 60 (the cross-sectional view in FIG. 1 is taken through the slot 60). The slot 60 is relatively narrow (e.g. less than one-eighth inch) and is in a plane parallel to the axis of the solenoid 12. The slot 60 reduces the circulation of eddy currents in the armature 22. The slot 60 also serves to aid fluid transfer from one end of the armature cavity 21 to the other end during operation. A bottom surface 62 of the slot 60 is positioned at an angle to the solenoid axis, as shown in FIG. 1, to provide maximum reduction of eddy currents, while still providing an uninterrupted contact surface for the pushpin 24, so that the transfer of solenoid force to the pushpin 24 is not impaired.

The pushpin 24 is preferably made of a material sold under the trademark Tantung G," the composition of which is:

Cobalt 45-50% Chromium 27-32% Tungsten l4-l9% Carbon 24% Tantalum or Columbium 27% Manganese l3% Iron 2-5% The pushpin 24 made of Tantung G has a hardness of about Rockwell 60 and is nonmagnetic. Such hardness is desirable to prevent mushrooming the ends of the pushpin 24 under impact. The magnetic flux which travels in the prior art magnetic steel pushpins represents about a 10 percent loss of solenoid force, and the pushpin 24 of the present invention eliminates this loss, while it maintains the hardness of a magnetic pin.

The stop 28 is shown in more detail in FIG. 3, and comprises a cylindrical body 70 having a retaining flange 72 on the bottom end thereof and having the axial bore 26 extending completely therethrough to accommodate the pushpin 24. The stop 28 is also provided with a relatively narrow slot 74, parallel to the solenoid axis, to reduce the circulation of eddy currents in the stop 28. The slot 74 extends through that portion of the stop 28 that is positioned axially within the yoke 16 and coil window 17. The top surface 76 of the stop is provided with a circular groove 78 to receive and hold a shading coil 79 (see FIG. 1), the purpose of which shading coil 79 is well known in the art.

The manual override 30 is shown in an enlarged cross-sectional view in FIG. 4, and comprises an end plate 80 fixedly secured to the armature tube 20 and provided with a cylindrical chamber 82 in which an override button 84 is positioned for axial sliding movement between a pair of snap rings 86 and 88. The end plate 80 is also provided with an axial bore 90, to accommodate an override pushrod 92 connected to the override button 84. The chamber-82 (and therefore the exterior of the solenoid I2) is sealed from fluid pressure in the solenoid 12 and in thevalve by means of an O-ring seal 94 held in place against the pushrod 92 by an O-ring retainer 96, which is, in turn, held in place by the snap ring 88. Leakage from the valve 10 through the solenoid 12 is thus prevented by the O- ring 94.

In operation, it'is clearly'seen that the armature 22 and the pushpin 26 can be causedto move to the right (as viewed in FIG. 1) to operate the valve 10 by manually pushing inwardly on the override button 84.

It has been discovered that the ratio of the length X of the coil 18 (FIG. 1), to the Ien'gth'Y of the working gap 50, is preferably in'the range of from about 3 to 6. If the ratio is much less than 3, then the flux leakage will increase to a point where the solenoids force will be significantly diminished. If the ratio is much greater than 6, then core losses will increase at a rate causingheating which more than offsets the small decrease in flux leakage.

It will thus be seen that, by virtue of the above-described preferred construction of the high-pressure push solenoid of the present invention, the objects and advantages set forth above are accomplished. For example, by eliminating the prior art O-ring seal on the pushpin to seal the solenoid from the valve, Applicant has eliminated the loss in solenoid force caused bythe drag of the O-ring on the pushpin. Also, by using a laminated yoke, a selectively magnetized and demagnetized armature tube, a slotted yoke, a semiaustenitic armature tube, a Tantung G pushpin, and by using a preferred ratio of coil length to length of working gap, much higher force and much cooler operation is achieved.

LOW-PRESSURE PUS H SOLENOID The low-pressure push solenoid of the present invention is identical to the highpressure push solenoid 12 described in detail above, except that in the low-pressure push solenoid, an armature tube is used that is made very thin and is all magnetic. The armature tube inthe low-pressure embodiment is about half as thick as the tube in the high-pressure embodiment, for example, in a -inch diameter armature tube, the tube wall thickness in the high-pressure solenoid is about 0.028 inch, while in the low-pressure solenoid it is about 0.014 inch. In the preferred construction the armature tube is heat treated to a very high strength to permit utilization of a very thin wall. The thin wall tube being of a magnetic material, offers relatively little resistance to magnetic flux passing radially thru the tube wall at locations thereof corresponding to sections 54 and 56 of FIG. l.'On the other hand, the same thin wall will both restrict the amount of flux passing axially thru the tube [at a location corresponding to section 58 (see FIG. 1)] and will also present a reduced path area for eddy current circulation in the tube. The tubeis preferably of a material having a low maximum magnetic permeability (about 200) to further inhibit the passage of flux linearly thru the tube at a location corresponding to section 58. The tube wall is sufficiently thin that the reduced permeability will not significantly inhibit the flux passing radially thru the tube wall at locations corresponding to sections '54 and 56. The utilization of the thin tube of the present invention will permit the reduction of solenoid holding current to avalue below that achieved with even a magnetic andnonmagnetic composite tube, and will provide a higher solenoid force level than a single-piece nonmagnetic tube.

Manufacturing costs of the thin tube are significantly less than that of the composite tube, and compares favorably with the nonmagnetic one-piece tube.

It has been found that the overall effect of any loss in efficiency due to some magnetic lines of force traveling linearly through the middle section of the tube (corresponding to section 58 of tube 20 of FIG. I)' is more than compensated for by the increased efficiency due to minimizing the hindrance of the magnetic field traveling radially through the outer sections of the armature tube (corresponding to sections 54 and 56 of PULL SOLENOID High-Pressure Pull Solenoid A With reference to the drawing, FIGS. 5 and 6 show one preferred high-pressure pull solenoid of the present invention, FIGS. 7 and 8 show another preferred high-pressure pull solenoid 150, and FIG. 9 shows a modified armature 194. It is to be particularly noted that by virtue of the. solenoid constructions shown in FIGS. 5 and 7, the prior artarmature stop has been eliminated. The elimination of the stop is very desirable because the stop was probably the highest producer of core losses in the solenoid, for the reason that the stop had to seal off hydraulic system pressure and therefore could not be laminated or even effectively slotted.

The pull solenoid 100 of FIG. 5 comprises: a yoke 102 defining a coil window 104, an annular coil 106 of known construction positioned in the coil window 104, and a hollow, cylindrical, armature tube 108, having an armature cavity 110 centrally positioned within the yoke 102' and the coil 106 and extending externally thereof through a circular opening 112 in a laminated sideplate 114 of the yoke 102. The coil 106 is provided with lead wires 116 as will be understood by one skilled in the art. A solid, cylindrical, nonlaminated, armature 1l8 is positioned within the armature cavity 110 in the armature tube 108, for axial sliding movement therein from the 'normal" position (shown in FIG. 9) to its energized position in contact with cap 139. The armature 118 includes a thin slot 120 extending at least through that'portion of the armature 118 that extends inside of the solenoid 100. The slot I20is parallel to, and extends at least through the axis of the armature 118. The slot 120 can also extend through that portion of the length of the armature 118 that exists outside of the solenoid 100 to provide a fluid passage to allow the necessary fluid transfer between a fluid valve (not shown) to which solenoid 100 is connected, and a working gap 122 in the armature cavity 110. The armature 118 does not need a separate groove for this purpose. A pull pin 124 is connected to the armature I18, and when the solenoid 100 is energized, the armature 118 is caused to move upwardly (as viewed in FIG. 5) through the working gap 122, thus actuating a fluid valve (not shown) connected to the pull pin 124.

With reference now to the exploded view of the yoke 102 in FIG. 6, the yoke 102 is formed from two separate, laminated parts; one part is a main body member 126 having the shape of a capital E and the other part is the straight, elongated sideplate 114. Both the member 126 and the sideplate 114 are formed from a vertical stack of thin, flat stampings 128 and 130, respectively. After the coil 106 has been placed in posi-' tion in the cavity 104 of member 126, the plate 112 is'attached to member 126, for example, by using a channel frame or by applying a layer of epoxy adhesive to the contacting surfaces of member 126 and plate 114, as will be understood by one skilled in the art. The yoke 102 does not require a slot suchas the slot 40 in the wound yoke 16 of FIG. 2, because the eddy current paths in yoke 102 are interrupted by insulative coating on each of the stampings l28 and 130.

A center leg extension 132 of member 126 serves (along with a cap 136 of the armature tube 108) as a stop for the armature 118, and because extension 132 is laminated, it allows the working gap 122 of the solenoid'l00 to be positioned in the middle of the coil window 104 without incurring ahigh eddy current loss penalty as would result if an unlaminated stop were used in place of the extension 132. The extension 132 should extend as far as possible into the coil window 104. Also, with the design of FIGS. 5 and 6, a shorter length of the armature tube 108 can be used. Because the armature tube 108 is nonlaminated and is therefore a generator of heat due to eddy currents within it, the shorter tube 108 results in a reduction of the amount of heat generated by the solenoid 100. Furthermore, a shorter armature tube 108 withstands the internal fluid pressure therein better than does a longer armature tube, and consequently the shorter tube 108 can be of a thinner wall thickness, thus resulting in improved overall solenoid performance. An additional advantage with this design is that a shorter armature 118 is used, and it can be nonlaminated.

The armature tube 108 comprises a tubular element 134 connected by welding or brazing to a magnetic steel cap 136 (see FIG. the wall of the tubular element 134 is just thick enough to properly sustain the operating pressure within the tube 108. The cap 136 has a thickness about twice that of the element 134 because the cap must not only sustain operating pressure but must also withstand impact of the armature 118. The tubular element 134 is made of a semiaustenitic material and has a nonmagnetic portion 138 and a magnetic portion 140 for the same reasons discussed in detail above with respect to the armature tube 20 of the push solenoid 12 of FIG. 1. The tube 108 can be connected to the sideplate 114 by known means, such as welding.

A top surface 142 of the armature 118 should be positioned, as shown in FIG. 5, in its normal position. In this position, the surface 142 is in line with an inner surface 144 of plate 112.

It has further been found, according to the present invention, that the width W (see FIG. 5) of the coil 106 should preferably bear a certain relationship to the length L (FIG. 5) of the working gap 122 of the solenoid 100. This preferred relationship is: 3L W 6L; in other words, the ratio W/L should be between 3 and 6. A preferred dimension is: W equals about one-fourth inch and L equals about one-sixteenth inch.

FIG. 7 shows another embodiment of a high-pressure pull solenoid 150 of the present invention. The solenoid 150 is similar in many respects to the solenoid 100 of FIG. 5 described above.

The pull solenoid 150 of FIG. 7 comprises a yoke 152 defining a coil window 154, an annular coil 156 (connected to lead wires 158) of known structure positioned in the coil window 154, and a hollow, cylindrical, armature tube 160 (comprising a tubular element 161 and a cap 163) centrally positioned within the yoke 152 and the coil 156, and extending externally from the yoke 152 through a circular opening 162 in a sidewall 164 of the yoke 152. As in the solenoid 100 of FIG. 5, the cap 163 is about twice as thick as element 161 and is welded or brazed thereto. A solid, cylindrical armature 166 connected to a pull pin 168 is positioned for axial sliding movement within a cylindrical armature cavity 170 of the armature tube 160. The solenoid 150 is shown in its unenergized or normal" position; upon energization, the armature 166 is caused to move upwardly (as viewed in FIG. 7) through a working gap 172 to actuate a fluid valve (not shown) such as valve ofFIG. 1.

With reference now to FIG. 8 showing an enlarged view of the armature 166, the armature 166 comprises a stack 174 of individual laminations 176, which stack 174 has been turned or ground cylindrical, and a magnetic sleeve 178 has been press-fitted over the entire length of the stack 174. The sleeve 178 constrains and holds the laminations 176 in their cylindrical configuration. A thin portion 180 of the length of the sleeve 178 is turned down to provide an extremely thin wall; e.g. about 0.007 inch to 0.009 inch thick.

The thin portion 180 of the sleeve 178 is that portion which is to be inside the magnetic circuit of the solenoid 150. The thin wall portion 180 produces a minimum of core losses and maintains the laminations 176 in their cylindrical configuration without the necessity for any additional bonding there between such as epoxy adhesive, to constrain and hold the laminations together. A thicker portion 182 of the sleeve 178 that is outside of the magnetic circuit is left thick for overall rigidity. The portion 182 is sufficiently long to provide a space for a pair of rivets 184 extending through the sleeve 178 and through all of the laminations 176. All rivets (e.g. 184) are outside of the magnetic circuit and therefore do not contribute to core loss. The portion 182 also preferably extends beyond an end 186 of the stack 174, to provide a good connection to the pull pin 168, such as by connecting the pull pin 168 to an end wall 188 of the sleeve 178. There is preferably a small clearance between the armature 166 and the inside surface of the tube 160 (of about 0.001 inch per side for a 400 c.p.s. solenoid); a longitudinal groove 190 (see FIG. 8) in armature surface 192 allows the necessary fluid flow between a fluid valve (not shown) and the working gap 172. The groove 190 can alternatively be a slot completely through the thickness of the sleeve portion 180.

The solenoid 150 of FIG. 7 is similar to the solenoid of FIG. 5 except that the yoke 152 of FIG. 7 comprises a single stack of rectangular laminations that form a simple rectangular coil window 154. The yoke 152 does not include the extension 132 of the yoke 102 of FIG. 5. The yoke 152 is less expensive to manufacture than the yoke 102 of FIG. 5, in part because the yoke 152 can be made in one piece. Both of the solenoids 100 and of FIGS. 5 and 7, respectively, can use any of the three armatures I18, 166 and 194 of FIGS. 5, 7, and 9 respectively.

FIG. 9 shows an alternative armature 194 for use in the solenoid 150 (FIG. 7). In the armature 194, a sleeve 196 (which need not be magnetic as is sleeve 178 of FIG. 8) is pressed only part way over the length of a stack 198 of individual laminations 200, which laminations 200 have been turned or ground cylindrical. The laminations 200 can also be bonded together, but because of the sleeve 196 the strength required of the bond, for example, an adhesive bond, is minimal. The armature tube of FIG. 7 will encompass the individual laminations 200 and help constrain and hold the laminations 200 in their proper relationship to each other. In operation, the part of the armature 194 not within the sleeve 196 is always inside the armature tube 160 of FIG. 7. If the laminations 200 are sufficiently thick and therefore rigid, no adhesive or other bonding is needed. A preferred construction is to use thin laminations 200 near the center of the stack 198 for good electrical performance, and thick laminations 200 near the outside of the stack 198 for rigidity. A pair of rivets 202 (only one is shown) extend through both the sleeve 196 and all of the laminations 200, to produce a single integral unit. The force of the armature 194 is applied (for example to a fluid valve similar to valve 10 of FIG. 1) via the sleeve 196 and not directly by the laminations 200. A pull pin (not shown) can be connected, for example, to a force-transmitting rivet 204 extending transversely across the interior diameter of the sleeve 196. The sleeve 196 and all rivets 202 lie outside the magnetic circuit and thus do not contribute to the core losses of the solenoid 150.

Low-Pressure Pull Solenoid The low-pressure pull solenoid of the present invention is identical to the high-pressure pull solenoids 100 and 150 of FIGS. 5 and 7 respectively, described above, except that in the low-pressure solenoid the armature tube is a very thin, very high strength, all magnetic tube as described above regarding the low-pressure push solenoid.

While the preferred embodiments described above relate to using the solenoids of the present invention as operators for hydraulic valves, it is to be noted that they are not limited to such use, but can be used with any kind of fluid (gas or liquid) valve, and in fact with other controls than fluid valves. While not shown above, the solenoids of the present invention can be completely encapsulated in a layer of epoxy resin. The armatures and armature cavities of the present invention are preferably circular in cross section, but can also have other shapes such as square, rectangular, and elliptical. Further, a push solenoid can be made using the construction shown in FIGS. 5 and 7 for pull solenoids; a laminated stop and a nonmagnetic pushpin would be used.

The invention has been described in detail above with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A push solenoid of the armature-in-tube type for use with hydraulic valves comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls of said yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair ofopenings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, said armature tube having an armaturechambertherein, an armature positioned in said armature chamber of said tube for axial sliding movement therein responsive toenergization of said coil, a stop connected to a bottom end of said tube andhaving an axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, fluid passage means extending through said stop into said chamber and fluid passage means extending through said'armature for fluid flow from one part of said chamber to another through said armature.

2. The apparatus according to claim 1 wherein the armature tube is made of a semiaustenitic material treated to be magnetic in the two sections thereof adjacent the yoke and to be nonmagnetic in a section thereof between said two sections.

3. A pull solenoid of the armature-in-tube type for use with hydraulic valves comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings and having an armature chamber therein, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said chamber of said tube for sliding movement thereinresponsive to energization of said coil, and fluid passage means in said armature for fluid flow into and out of said chamber through said armature.

4. The apparatus according to claim 3 wherein said yoke is substantially rectangular and comprises a plurality of stamped laminations lying in a plane parallel to the axis of the solenoid, said yoke having two short end walls'and two longer sidewalls and wherein said yoke opening is a circular opening and wherein said armature tube is cylindrical and is connected to said yoke at said single circular-opening in said yoke.

5. The apparatus according to claim 3 wherein the tube is made of semiaustenitic material'treated to be magnetic in one section adjacent the yoke and nonmagnetic in a section adjacent the coil.

6. The apparatus according to claim 3 wherein said yoke includes a central, axial, laminated extension extending inwardly of said coil opening and extending part way through said coil opening.

7. An armature for a solenoid comprising a stack of individual, flat, laminations, parallel to the armature axis, said stack having a cylindrical outer surface concentric to the armature axis and a cylindrical, metal sleeve tightly fitting on said surface over at least a portion of the axial length of said stack.

8. The apparatus according to claim 7 including an armature pull pin connected to said sleeve.

9. The apparatus according to claim 7 wherein said sleeve includes a first, very thin portion extending over that length of said armature positioned inside of the solenoid.

10. A hollow, solenoid armature tube made of semiaustenitic material treated to be magnetic in one section and nonmagnetic in a second section thereof.

11. In an AC solenoid including an "annular coil and an axially movable arrnature therein movable across a working gap when the coil is energized, the improvement wherein the ratio of the coil length to the length of the working gap is between about 3 and 6.

12. In a low-pressure AC solenoid of the armature-in-tube type including a yoke, a coil in the yoke, an armature tube inside the coil and yoke, and an armature movably mounted in the armature tube, the improvement wherein said armature tube is all magnetic, heat treated to increase its strength, and has a wall thickness of about 0.014 inch.

13. A push solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein andhaving a pair of openings in opposed sidewalls of said yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, an armature positioned in said tube for axial sliding movement therein responsive to energization of said coil, a stop connected to a bottom end of said tube and having an'axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, meansfor sealing thetop end ofsaid armature tube against leakage of fluid therefrom, and wherein the ratio of the length of the coil to the length of the working gap is between about 3 and 6.

14. A push solenoid of the 'armature-in-tube type comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls ofsaid yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, an armature positioned in said tube for axial sliding movement therein responsive to energization of said coil, a stop connected to a bottom end of said tube and having an axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, and wherein said yoke includes slots to prevent the circulation of eddy currents in paths concentric to the solenoid axis.

15. A push solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls of said yoke,-an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, an armature positioned in said tube for axial siding movement therein responsive to energization of said coil, a stop connected to a bottom end of said tube and having an axial bore extending therethrough, a pushpin positioned against saidarmature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, and wherein said armature includes a thin slot therethrough parallel to the solenoid axis, the depth of the slot being at an angle to the solenoid axis to provide a solid, central portion for contacting said pushpin.

16. A push solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls of said yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, an armature positioned in said tube for axial sliding movement therein responsive to energization of said coil, a stopco'nnected to a bottom end of said tube and having an axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, and including a manual override positioned adjacent the top end of said tube and including an override button connected to an override pushrod, said pushrod being movable into contact with said armature for manually moving said armature to its energized position, and wherein said fluid seal means comprises an O- ring seal in sealing engagement with said override pushrod.

17. A pull solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said tube for sliding movement therein responsive to energization of said coil, and wherein said closed end of said armature tube is positioned immediately adjacent the inside surface of the other sidewall of said yoke.

18. A pull solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said tube for sliding movement therein responsive to energization of said coil, and wherein the ratio of the length of said coil to the length of the working gap is between about 3 and 6.

19. A pull solenoid of the annature-in-tube type comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said tube for sliding movement therein responsive to energization of said coil, wherein said yoke includes a central, axial, laminated extension extending inwardly of said coil opening and extending part way through said coil opening, wherein said armature is solid and includes a slot parallel to the axis of the solenoid for reducing the circulation of eddy currents therethrough, and wherein the top end of said armature is in line with the inner surface of the bottom sidewall of the yoke, when the armature is in its normal" position.

. 20. A pull solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said tube for sliding movement therein responsive to energization of said coil, wherein said armature comprises a cylindrical stack of flat, individual laminations, parallel to the axis of said solenoid, and including a sleeve tightly fitting over at least a portion of the axial length of said stack to hold said laminations together.

21. The apparatus according to claim 20 including an armature pull pin connected to said sleeve.

22. The apparatus according to claim 20 wherein said sleeve includes a first, very thin portion extending over that length of said armature positioned inside of said yoke.

23. The apparatus accordlng to claim 22 wherein said sleeve also includes a second thicker portion covering that length of said armature that remains outside of said yoke.

24. An armature for a solenoid comprising a stack of individual laminations, parallel to the armature axis, and a sleeve tightly fitting over at least a portion of the axial length of said stack, wherein said sleeve includes a first, very thin portion extending over that length of said armature positioned inside of the solenoid, and wherein said sleeve also includes a second thicker portion covering that length of said stack that remains outside of said solenoid.

25. The method comprising forming the armature tube of an armature-in-tube type of solenoid of semiaustenitic material, and treating said tube to be magnetic in at least one section and nonmagnetic in at least one other section.

26. The method comprising forming a solenoid an'nature as a cylindrical stack of flat, individual laminations parallel to the solenoid axis and with the cylindrical surface concentric with said axis, and holding said laminations together, in the portion thereof to be received within a solenoid, by providing a tightly fitting cylindrical sleeve over at least that portion of said stack.

27. The method comprising employing for the ratio of coil length to the length of the working gap, in a push-type solenoid, a value between about 3 and 6.

28. The method comprising forming the armature tube of a low-pressure AC solenoid of the armature-in-tube type, of very thin magnetic material having a wall thickness of about 0.014 inch, and heat treating said material to increase its strength. 

1. A push solenoid of the armature-in-tube type for use with hydraulic valves comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls of said yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, said armature tube having an armature chamber therein, an armature positioned in said armature chamber of said tube for axial sliding movement therein responsive to energization of said coil, a stop connected to a bottom end of said tube and having an axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, fluid passage means extending through said stop into said chamber and fluid passage means extending through said armature for fluid flow from one part of said chamber to another through said armature.
 2. The apparatus according to claim 1 wherein the armature tube is made of a semiaustenitic material treated to be magnetic in the two sections thereof adjacent the yoke and to be nonmagnetic in a section thereof between said two sections.
 3. A pull solenoid of the armature-in-tube type for use with hydraulic valves comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings and having an armature chamber therein, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said chamber of said tube for sliding movement therein responsive to energization of said coil, and fluid passage means in said armature for fluid flow into and out of said chamber through said armature.
 4. The apparatus according to claim 3 wherein said yoke is substantially rectangular and comprises a plurality of stamped laminations lying in a plane parallel to the axis of the solenoid, said yoke having two short end walls and two longer sidewalls and wherein said yoke opening is a circular opening and wherein said armature tube is cylindrical and is connected to said yoke at said single circular opening in said yoke.
 5. The apparatus according to claim 3 wherein the tube is made of semiaustenitic material treated to be magnetic in one section adjacent the yoke and nonmagnetic in a section adjacent the coil.
 6. The apparatus according to claim 3 wherein said yoke includes a central, axial, laminated extension extending inwardly of said coil opening and extending part way through said coil opening.
 7. An armature for a solenoid comprising a stack of individual, flat, laminations, parallel to the armature axis, said stack having a cylindrical outer surface concentric to the armature axis and a cylindrical, metal sleeve tightly fitting on said surface over at least a portion of the axial length of said stack.
 8. The apparatus according to claim 7 including an armature pull pin connected to said sleeve.
 9. The apparatus according to claim 7 wherein said sleeve includes a first, very thin portion extending over that length of said armature positioned inside of the solenoid.
 10. A hollow, solenoid armature tube made of semiaustenitic material treated to be magnetic in one section and nonmagnetic in a second section thereof.
 11. In an AC solenoid including an annular coil and an axially movable armature therein movable across a working gap when the coil is energized, the improvement wherein the ratio of the coil length to the length of the working gap is between about 3 and
 6. 12. In a low-pressure AC solenoid of the armature-in-tube type including a yoke, a coil in the yoke, an armature tube inside the coil and yoke, and an armature movably mounted in the armature tube, the improvement wherein said armature tube is all magnetic, heat treated to increase its strength, and has a wall thickness of about 0.014 inch.
 13. A push solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls of said yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, an armature positioned in said tube for axial sliding movement therein responsive to energization of said coil, a stop connected to a bottom end of said tube and having an axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, and wherein the ratio of the length of the coil to the length of the working gap is between about 3 and
 6. 14. A push solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls of said yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, an armature positioned in said tube for axial sliding movement therein responsive to energization of said coil, a stop connected to a bottom end of said tube and having an axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, and wherein said yoke includes slots to prevent the circulation of eddy currents in paths concentric to the solenoid axis.
 15. A push solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls of said yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, an armature positioned in said tube for axial siding movement therein responsive to energization of said coil, a stop connected to a bottom end of said tube and having an axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, and wherein said armature includes a thin slot therethrough parallel to the solenoid axis, the depth of the slot being at an angle to the solenoid axis to provide a solid, central portion for contacting said pushpin.
 16. A push solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having a pair of openings in opposed sidewalls of said yoke, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said pair of openings, a hollow armature tube extending through said pair of openings in said yoke and also through said central opening, an armature positioned in said tube for axial sliding movement therein responsive to energization of said coil, a stop connected to a bottom end of said tube and having an axial bore extending therethrough, a pushpin positioned against said armature and extending exteriorly of said solenoid through said bore in said stop, means for sealing the top end of said armature tube against leakage of fluid therefrom, and including a manual override positioned adjacent the top end of said tube and including an override button connected to an override pushrod, said pushrod being movable into contact with said armature for manually moving said armature to its energized position, and wherein said fluid seal means comprises an O-ring seal in sealing engagement with said override pushrod.
 17. A pull solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said tube for sliding movement therein responsive to energization of said coil, and wherein said closed end of said armature tube is positioned immediately adjacent the inside surface of the other sidewall of said yoke.
 18. A pull solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said tube for sliding movement therein responsive to energization of said coil, and wherein the ratio of the length of said coil to the length of the working gap is between about 3 and
 6. 19. A pull solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said tube for sliding movement therein responsive to energization of said coil, wherein said yoke includes a central, axial, laminated extension extending inwardly of said coil opening and extending part way through said coil opening, wherein said armature is solid and includes a slot parallel to the axis of the solenoid for reducing the circulation of eddy currents therethrough, and wherein the top end of said armature is in line with the inner surface of the bottom sidewall of the yoke, when the armature is in its ''''normal'''' position.
 20. A pull solenoid of the armature-in-tube type comprising a laminated yoke having a coil window therein and having an opening in a bottom sidewall only thereof, an annular coil positioned in said coil window, said coil having a central opening therethrough concentric with said opening in said yoke, a hollow armature tube positioned in said openings, said tube being open at a bottom end thereof adjacent said opening in said one sidewall of said yoke and being closed at the top end thereof adjacent the other sidewall of said yoke, an armature positioned in said tube for sliding movement therein responsive to energization of said coil, wherein said armature comprises a cylindrical stack of flat, individual laminations, parallel to the axis of said solenoid, and including a sleeve tightly fitting over at least a portion of the axial length of said stack to hold said laminations together.
 21. The apparatus according to claim 20 including an armature pull pin connected to said sleeve.
 22. The apparatus according to claim 20 wherein said sleeve includes a first, very thin portion extending over that length of said armature positioned inside of said yoke.
 23. The apparatus according to claim 22 wherein said sleeve also includes a second thicker portion covering that length of said armature that remains outside of said yoke.
 24. An armature for a solenoid comprising a stack of individual laminations, parallel to the armature axis, and a sleeve tightly fitting over at least a portion of the axial length of said stack, wherein said sleeve includes a first, very thin portion extending over that length of said armature positioned inside of the solenoid, and wherein said sleeve also includes a second thicker portion covering that length of said stack that remains outside of said solenoid.
 25. The method comprising forming the armature tube of an armature-in-tube type of solenoid of semiaustenitic material, and treating said tube to be magnetic in at least one section and nonmagnetic in at least one other section.
 26. The method comprising forming a solenoid armature as a cylindrical stack of flat, individual laminations parallEl to the solenoid axis and with the cylindrical surface concentric with said axis, and holding said laminations together, in the portion thereof to be received within a solenoid, by providing a tightly fitting cylindrical sleeve over at least that portion of said stack.
 27. The method comprising employing for the ratio of coil length to the length of the working gap, in a push-type solenoid, a value between about 3 and
 6. 28. The method comprising forming the armature tube of a low-pressure AC solenoid of the armature-in-tube type, of very thin magnetic material having a wall thickness of about 0.014 inch, and heat treating said material to increase its strength. 